Infrared detectors, useful for detecting the location of objects by the infrared energy radiating from the objects, are known in the prior art. For example, infrared detectors are used to guide weapons to a target, particularly ground-to-air and air-to-air missiles, and are used in scopes to enhance night-time observation.
Infrared up-convertors are solid-state electronic arrays that receive an infrared image on one surface and emit a corresponding image at a selected wavelength from another surface. If an up-convertor is disk-shaped, an infrared image is focused onto one side and the other side emits a corresponding image, usually at a shorter wavelength. In some applications the emitted image is directly viewed by a human observer or a camera. In other applications the image may be further processed by a device such as a computer.
Infrared-sensitive devices, including up-convertors, are preferably operated at very low temperatures such as 80.degree. Kelvin. At such low temperatures, up-convertors operate most effectively and have increased sensitivity and increased signal-to-noise ratio. Because infrared detectors are typically installed on aircraft, missiles, and other mobile devices and because the detector itself is often gimbal-mounted for tracking objects, the cooling apparatus must not only provide low temperatures but must be relatively small in mass and size and must be able to operate in varied attitudes. It has been found that a cryostat based on the Joule-Thomson effect can meet these requirements.
In a Joule-Thomson cryostat a flow of high-pressure coolant gas, such as nitrogen or argon at, for example, 2000-3000 pounds per square inch, is throttled. The adiabatic cooling upon expansion converts the coolant to a liquid state, or possibly even to a solid state. The low temperature of the coolant is then used to cool the infrared detector. Typically, the expansion-cooled outgoing coolant is used to cool the incoming coolant.
Previously, however, it has not been possible to use up-convertors to their fullest advantage because there was no adequate cooling method which did not intefere with the incoming or outgoing energy; that is, the prior art cooling apparatuses blocked or obscured the path of light to or from the up-convertor. In the prior art the coolant has been used to cool the entire surface of the up-convertor convectively. This has several disadvantages. In convective cooling the light energy must pass thru the coolant, therefore a coolant with a refractive index of anything other than 1 will disturb the focusing of the image. Additionally, turbulence, swirls, and eddies in the convective coolant will cause distortion of the image. Also, near this temperature and pressure it is often very difficult to control the exact nature of the coolant, and slight temperature and pressure variation may produce localized gas, liquid, "ice" or "snow" with the concomitant differing physical qualities which greatly disrupt the image path and disturb the image.
Therefore is is desirable to have a cooling apparatus for an infrared up-convertor which does not obscure the path of energy to or from the up-convertor.
It is also desirable that such a device be small in size and mass and be operable in various attitudes.